A COTDR (Coherent Optical Time Domain Reflectometer, coherent optical time domain reflectometer) and an OTDR are usually used for locating a line fault (such as fiber cut) of an optical fiber cable. Both the OTDR and the COTDR send a probe pulse to the fiber periodically, where the probe pulse undergoes Rayleigh scattering at each point in the fiber. The COTDR and OTDR then extract a Rayleigh-scattered signal from a received optical signal scattered back from the fiber, and then perform analysis according to the Rayleigh-scattered signal, to implement fiber fault location. Compared with the common OTDR, the COTDR has advantages such as long location distance. However, because a coherent technology is adopted to receive and process the Rayleigh-scattered signal, it is required that a line width of a light source of the COTDR should be narrow, and usually the line width is required to be within a range of 10 KHz to 100 KHz.
Since the optical wave of the probe pulse has a single frequency component, a Rayleigh-scattered signal scattered back from each point in the fiber has a same frequency, which thereby causes Rayleigh-scattered signals scattered back from some points to undergo destructive interference or constructive interference when arriving at the OTDR or COTDR. As a result, the Rayleigh-scattered signals demonstrate a noise-like characteristic, that is, present certain randomness, and such a noise is referred to as a coherent fading noise. The coherent fading noise causes the COTDR and OTDR to be incapable of distinguishing a fiber fault, and thereby fail to accurately locate the fiber fault. Therefore, such a noise-like characteristic needs to be reduced by certain means, so as to improve the fiber fault resolution and location accuracy of the COTDR and OTDR.
The prior art provides a solution of directly adopting a technology of changing a drive pulse shape of a laser device to reduce the coherent fading noise of an OTDR system, which specifically is that a drive control circuit generates a slowly changing modulation pulse to drive a pulse modulator to perform modulation control on the laser device, so that a probe pulse output by the laser device include optical waves of different frequency components, thereby achieving an objective of reducing the coherent fading noise.
However, the prior art results in broadening of a frequency spectrum of the optical waves output by the light source, which does not meet a requirement of the COTDR for narrow line width of the light source. When being applied to the COTDR, the prior art not only fails to improve the fiber fault resolution and location accuracy of the COTDR, but also degrades working performance of the COTDR.